1. Field of the Invention
This invention relates to a method and a system for removing impurities from sewage.
2. State of the Art
Sewage often contains a number of impurities which include materials such as sugars and other carbohydrates, and proteins and other forms of nitrogen. Many of these impurities are decomposed by micro-organisms, and oxygen in the sewage is used by the micro-organisms to accomplish the decomposition. Therefore, the impurities are generally called oxygen-demanding, and they are said to produce a biochemical or nitrogenous oxygen demand, depending upon their chemical composition.
Ammonia and many chemicals containing organic nitrogen are decomposed and oxidized according to the process of nitrification to produce compounds containing nitrates. Often, it is desirable to remove the nitrates from the sewage, and this is accomplished according to the process of denitrification.
A conventional sewage treatment system, called an orbital system, is taught in U.S. Pat. No. 3,510,110. Another orbital system is taught in a paper entitled, "Nitrification and Denitrification in Carrousel Systems," by Anton T. van der Geest and William C. Witvoet, presented at the annual meeting of the Water Pollution Control Federation in 1975. The orbital system taught in the paper by van der Geest and Witvoet operates to accomplish nitrification and denitrification and includes a control system to control the extent to which nitrification and dentrification take place. However, the control system is unable to operate effectively if the characteristics of the sewage fluctuate substantially.